In SiC power MOSFETs fabricated using SiC having 4H crystallinity, channel mobility is known to be lower than that of theoretical prediction.
In the SiC power MOSFET, as a cause for lowering the channel mobility, an interface state or surface roughness at a channel interface, the influence of phonon scattering, or the influence of Coulomb scattering based on fixed charge is considered as a main factor. In particular, when a SiC surface is thermally oxidized to form a gate insulating film (silicon dioxide), a layer containing excessive carbon (hereinafter represented as a carbon-excess layer) is formed as an interface layer between SiC and silicon dioxide in the vicinity of a channel surface, and this carbon-excess layer is known to reduce the carrier mobility in an inversion layer channel (refer to NPL 1). Chokawa et al. have reported that the application of mechanical stress due to oxidation to SiC crystal causes the generation of this carbon-excess layer (refer to NPL 2). FIG. 1 shows an explanatory view of the generation mechanism of the carbon-excess layer. When oxygen combines with silicon in crystal by oxidation, deformation occurs in a crystal layer in the vicinity of an interface. Therefore, carbon atoms in SiC crystal below the interface approach each other, form carbon-carbon (C—C) bonds, and thus are stabilized. Since this bond is stable, the formed carbon-carbon bonds continue to remain through a subsequent process, and thereby form the carbon-excess layer. Carbon-carbon bonds present in the vicinity of a channel interface form a trap level and distort an electric potential, thereby causing mobility degradation.
Okada et al. have reported based on analysis using an X-ray photoelectron spectroscopy that carbon-carbon bonds are formed in the vicinity of an interface when a SiC crystal surface having this 4H crystallinity is thermally oxidized (refer to NPLs 3 and 4). It is shown that a large number of carbon-carbon bonds exceeding 10% of silicon-carbon bonds are present by oxidation in a region 2 nm or less below the interface. Moreover, when this carbon-excess layer is formed, this change can be seen by analyzing a crystal structure of the interface using a high-power transmission electron microscope. It is known that when a gate insulating film is formed on a silicon substrate and its interface is observed by the same method, an interface between silicon and an oxide film is formed within the range of about an atomic step. In contrast to this, at an interface between SiC and an oxide film, since a crystal structure is deformed due to carbon-carbon bonds, an interface region is observed to be spread over two to three steps.